The present invention relates to an apparatus and method for assessing loads on adjacent bones, and is particularly directed to an apparatus and method for providing an in vivo assessment of loads on adjacent bones to be fused together.
It is known to use surgical procedures to stabilize a fractured bone or repair a problematic interaction of adjacent bones. For example, spinal surgery is frequently performed to stabilize a problematic portion of the spine and relieve pain. Often, the vertebrae in the problematic portion of the spine are fused together with a bone graft in order to achieve the stabilization. Because the bone fusion takes time (six months or more on average), spinal implants (often referred to as fixation instrumentation), such as rods, clamps, and plates, are typically implanted and used to secure the vertebrae while the fusion of the bone graft takes place.
During the months while the arthrodesis is occurring, it is desirable to monitor the progress of the bony incorporation, or bone-ingrowth, of the graft. Known methods for examining the bony incorporation include radiographic evaluation, magnetic resonance imaging, and computerized tomography. All of these techniques provide a snapshot of the progress of the bony incorporation, but do not provide accurate, continuous, real-time information to the patient and physician. Without the ability to accurately and continuously assess the bony incorporation, pseudoarthrosis (non-healed bone fusion) may occur unbeknownst to the physician. Such pseudoarthrosis may cause post-operative pain for the patient and necessitate additional surgery. If the fusion progress could be assessed continuously or on-demand during the post-operative period by assessing the loads on the fixation instrumentation, it may be possible to appropriately time additional surgery or even avoid additional surgery.
In a related manner, it is also desirable to assess the biomechanical performance of implanted spinal fixation instrumentation during the post-operative period while bone fusion is occurring. Both in vitro and in vivo biomechanical testing of fixation instrumentation has been done in the past, but with limited success. Current in vitro testing of fixation instrumentation typically subjects cadaveric vertebrae and implantable instrumentation to various axial and torsional loading parameters on a hydraulic testing apparatus. Unfortunately, the use of non-living cadaveric tissue can introduce significant error into the test data.
Previous attempts at in vivo biomechanical testing of spinal fixation instrumentation have been done primarily using animals (quadrapeds), but some limited testing has been done with humans. In one of the in vivo human tests performed to date, sensors that were placed on the implanted spinal instrumentation utilized wires to carry data percutaneously (through the skin) from the sensors to a data monitoring unit outside the human body. The use of wires or other type of electrical or optical connection extending through the skin provides a significant risk of infection and is not suitable for long-term testing as there is a high risk of wire breakage.
Another problem encountered with the in vivo testing that has been done is failure of a sensor, such as a strain gauge, or the sensor wiring which has been known to break, corrode, or debond within four months of in vivo implantation. While attempts have been made to use telemetry to transmit data from sensors implanted in transcranial applications to an external monitoring device, a need exists for an implantable, telemetered sensor arrangement for spinal or other orthopedic applications that could survive a minimum of a year.
Microelectromechanical systems, or MEMS, refers to a class of miniature electromechanical components and systems that are fabricated using techniques originally used in the fabrication of microelectronics. MEMS devices, such as pressure sensors and strain gauges, manufactured using microfabrication and micromachining techniques can exhibit superior performance compared to their conventionally built counterparts and are resistant to failure due to corrosion, etc. Further, due to their extremely small size, MEMS devices can be utilized to perform functions in unique applications, such as the human body, that were not previously feasible using conventional devices.
The present invention is an apparatus for providing an in vivo assessment of loads on adjacent bones. The apparatus comprises a body for insertion between the adjacent bones. At least one sensor is associated with the body. The at least one sensor generates an output signal in response to and indicative of a load being applied to the body through the adjacent bones. At least one telemetric device is operatively coupled with the at least one sensor. The least one telemetric device is operable to receive the output signal from the at least one sensor and to transmit an electromagnetic field (EMF) signal dependent upon the output signal.
In accordance with one embodiment of the invention, the body comprises an implant for helping the adjacent bones to fuse together. The implant comprises a bone graft. In accordance with another embodiment of the invention, the body comprises a fusion cage for insertion between an adjacent pair of vertebrae. In accordance with yet another embodiment, the body comprises a prosthetic device for preserving motion between adjacent bones.
According to various features of the invention, the at least one sensor comprises a pressure sensor, a load cell, and/or at least one strain gauge.
According to another aspect of the present invention, an apparatus for providing an in vivo assessment of loads on adjacent bones comprises a body for insertion between the adjacent bones and sensor means for sensing a load being applied to the body through the adjacent bones. The sensor means generates a corresponding output signal in response to and indicative of a sensed load. First circuit means is operatively coupled with the sensor means for receiving the output signal from the sensor means. The first circuit means includes antenna means for receiving energy to power the first circuit means and the sensor means and for transmitting an EMF signal dependent upon the output signal.
In according with another feature of the invention, the apparatus further comprises second circuit means for transmitting energy to power the first circuit means and the sensor means and for receiving the EMF signal. The second means is disposed remote from the first circuit means.
According to yet another aspect of the present invention, an apparatus for providing an in vivo assessment of loads on and motion of one or more bones comprises a member for placement adjacent a bone and at least one sensor associated with the member. The at least one sensor generates an output signal in response to and indicative of a load being applied to the member through the bone. At least one telemetric device is operatively coupled with the at least one sensor. The at least one telemetric device is operable to receive the output signal from the at least one sensor and to transmit an EMF signal dependent upon the output signal.
According to various embodiments of the present invention, the member comprises an implant for helping adjacent bones fuse together, such as a fusion cage, a fixation plate, and/or a bone graft. Alternatively, the member comprises a prosthetic device for preserving motion between adjacent bones.
According to still another aspect of the present invention, an apparatus for providing an in vivo assessment of loads on and motion of one or more bones comprises at least one sensor attached to a bone. The at least one sensor generates an output signal in response to and indicative of a load on the bone. At least one telemetric device is operatively coupled with the at least one sensor. The at least one telemetric device is operable to receive the output signal from the at least one sensor and to transmit an EMF signal dependent upon the output signal.
The present invention also provides a method for in vivo assessing the loads on adjacent bones to be fused together. According to the inventive method, a body for insertion between the adjacent bones is provided. The body is instrumented with at least one sensor for sensing a load on the body and for generating an output signal indicative of a sensed load. At least one telemetric device is operatively coupled with the at least one sensor to receive the output signal and to transmit an EMF signal dependent upon the output signal. The body is implanted between the adjacent bones. The EMF signal from the at least one telemetric device is then monitored.